1. Field of the Invention
The invention concerns integrated circuits and, more particularly, a way of making a threshold comparator that is immunized against noise, especially the noise constituted by the variations in the supply voltage Vcc of the integrated circuit or the variations in electrical ground potential.
2. Description of the Prior Art
In a threshold comparator, the threshold beyond which the comparator changes over to one direction or the other depends, to a greater or to a lesser degree, on the supply voltage of the comparator or, more exactly, on the difference between this voltage and the ground. But there may be fluctuations in the supply voltage or in the ground potential (this ground potential being variably distributed through the integrated circuit). These fluctuations, known as supply circuit noise, are a hindrance to the working of the comparator.
This is especially the case, for example, in a circuit for counting half waves of an alternating signal or in a circuit counting the instances when a periodic signal crosses a determined threshold. The comparator is used in this case to detect the instances when a determined threshold is crossed. However, if the reference voltage varies during the comparison, there is the risk of a double count when only one crossing of the threshold ought to be detected. The count is then false, and this is a very major drawback.